Mirienna was a female human who resided on the planet of Zeffo during the Imperial Era. During the Galactic Empire's occupation of Zeffo for the sake of Project Auger, the residents of a Zeffo village were displaced. To allow Mirienna to flee with her daughter, Sienna, her husband, Ostar, drew away several stormtroopers. This attempt resulted in her husband's death, and Mirienna subsequently aligned herself with Saw Gerrera's Partisans, sending her daughter to the safety of Chandrila, though she later resolves to reunite with Sienna.
Living in a village located on the planet Zeffo was Mirienna, a human, along with her husband, Ostar, and their infant daughter, Sienna. Around 14 BBY, the Galactic Empire took control of Zeffo to implement Project Auger, which led to the displacement of the village's inhabitants. Ostar created a diversion by distracting a squad of stormtroopers, giving Mirienna the opportunity to escape with Sienna, resulting in the family's separation and the loss of a bag in the village. Mirienna and Sienna sought refuge in a cave tunnel that connected the village to a Derelict Hangar through a mountain. Inside the cave, Mirienna used a piece of fabric as a blanket to keep a crying Sienna warm, but left the blanket in the cave before escaping Zeffo without Ostar, not knowing if her husband was alive or dead.
Following this, Mirienna arranged for Sienna to live with her father on Chandrila. Mirienna then traversed the galaxy to fight against the Empire and joined Saw Gerrera's rebel Partisans. She participated in battles alongside Gerrera on Kashyyyk, where the Empire had enslaved the native Wookiees, and contributed to the successful liberation of Wookiee prisoners from an Imperial Refinery, aided by the fugitive Jedi Cal Kestis.
Mirienna received an encoded message from her father confirming Sienna's safe arrival on Chandrila. On the captured Cargo Pad 119-Grek, Kestis approached her, inquiring about her well-being, and Mirienna shared the news, describing it as the best she had received since arriving on Kashyyyk. Kestis then revealed that he had experienced visions of Ostar on Zeffo and discovered his deceased body, confirming his fate. Despite Kestis's apology for the bad news, Mirienna expressed gratitude for knowing the truth.
Upon Kestis's return to Kashyyyk, having secured a meeting with Tarfful, Mirienna expressed her relief at his safety and informed him that Mari Kosan and Choyyssyk had created a map to the rendezvous point for him. She verified that Gerrera had retreated off-world and that their casualties had reached unsustainable levels. While acknowledging that the situation was dire under the Empire, she encouraged Kestis, emphasizing that his quest still had a chance of success. Thanking him for his contributions to the fight, she announced her decision to leave Mari Kosan's cell to raise Sienna while continuing to oppose the Empire through other means.
Mirienna was a courageous warrior, but she longed for her daughter while fighting with Gerrerra. After being separated from Ostar, she hoped that he had escaped the Imperials, but when Kestis confirmed his death, she was relieved to know the truth. She believed that the Empire should not be allowed to destroy any more worlds. After learning about her husband's death, she decided to leave the battlefield to raise her daughter while continuing to find other ways to fight. Mirienna had dark skin, black hair, and brown eyes.
During her time on Kashyyyk, Mirienna wore a yellow and tan jacket with gray pants, black boots, and a brown backpack. Her weapon of choice was a DC-15A blaster carbine.
Mirienna made her debut in the 2019 video game Star Wars Jedi: Fallen Order, developed by Respawn Entertainment.